Asphalt cement concrete interlayer system for reflective crack relief

ABSTRACT

An asphalt-cement concrete (“ACC”) interlayer formed of a plant-mix material reinforced with aramid fibers, deposited at a thickness of at least one inch (1″) over a Portland-cement concrete (“PCC”) or ACC base, can extend the service life of a hot-mix asphalt (“HMA”) surface layer installed over the interlayer by retarding or preventing “reflected” cracks—cracks in the surface layer that correspond to cracks, damage and irregularities in the PCC or ACC base. When the surface layer&#39;s useable life has expired, it can be removed and replaced, and the interlayer can continue to protect the new surface layer.

CONTINUITY AND CLAIM OF PRIORITY

This is an original U.S. patent application.

FIELD

The invention relates to heterogenous layered structures for asphaltcement concrete. More specifically, the invention relates to anengineered asphalt cement concrete interlayer for improving theperformance and longevity of a multi-layer asphalt paving system.

BACKGROUND

Asphalt Cement Concrete (“ACC,” or often just “asphalt”) is widely usedas a paving material to surface roads, runways and parking lots. By someestimates, up to 90% of all such surfaces are made with ACC. A basicasphalt concrete comprises asphalt (also known as bitumen), ahighly-viscous or semi-solid form of petroleum; and aggregates such asstone, sand or gravel, in about a 1:19 ratio (5% asphalt, 95%aggregate). The ingredients are heated, mixed, spread on the surface tobe paved (often an earthen, stone or crushed-rock bed) and compacted toform ACC.

One common application for ACC is as an overlay to restore deterioratingPortland cement concrete (“PCC”) surfaces (old ACC surfaces can also berepaired with a new ACC overlay). In these repairs, the ACC is of a typeknown as hot mix asphalt (“HMA”). Overlays protect the existing pavementstructure and provide a new, smoother, skid-resistant riding surface.According to American Society of Civil Engineers (ASCE) 2017 reportcard, the U.S. highway system has been underfunded, resulting in $836billion backlog of capital needs of which $420 billion is to repair ourhighways.

However, PCC is a rigid pavement, while hot mix asphalt (HMA) is aflexible pavement. This mismatch of moduli (strengths) results in cracksforming in the HMA overlay as a result of the underlying concretepavement joints (and other irregularities and damage in the PCC). Thecracks are commonly called “reflective cracks” (or “reflectivecracking”) as shown in FIG. 2. Reflective cracks usually begin to appearin the new HMA surface within one or two years, depending on thethickness of the overlay. It is common to see all cracks reflect throughthe new overlay within three to five years. It is common to see cracksre-appear at a rate of 25% per year after the first year. As a generalrule of thumb, it is commonly said that cracks move upward at about oneinch per year.

Asphalt rubber interlayers, fabrics, and other methods have been used toaddress this problem. The performance of these methods has variedgreatly. An Army Corps of Engineers study concluded that while some ofthe methods work well over flexible pavements in warm climates, theperformance has been generally unsatisfactory in cold climates, and theuse of asphalt rubber membranes is not effective over PCC in anyclimate. Most of the existing reflective crack relief methods and theirspecifications mainly address tension, that is, controlling horizontalmovement at the crack or joint interface. But there are actually threeforces that need to be addressed: horizontal non-load movement(tension), vertical (shear) load induced movements, and bending orparallel movement under laterally unstable conditions. The shearmovement is usually the movement that causes the quickest reflectivecracks. This movement is from a lack of load transfer in the PCC jointand usually called a moving joint.

New approaches and new structures for retarding or preventing reflectivecracking in PCC (and ACC) resurfacing operations may be of significantvalue in this industry.

SUMMARY

Embodiments of the invention provide an interlayer of asphalt-cementconcrete comprising aramid fibers in a plant-mix material, installedover a PCC (or ACC) surface to be restored. The aramid-modifiedinterlayer can withstand instability from irregularities in theunderlying pavement and reduce the tendency of those irregularities tomanifest as cracks and other damage in the travel surface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a typical layer stack-up of an embodiment of the invention.

FIG. 2 shows images of a prior-art pavement repair.

FIG. 3 is a flow chart outlining a method according to an embodiment.

FIG. 4 is a flow chart outlining another method according to anembodiment.

DETAILED DESCRIPTION

Basic asphalt cement concrete can be formulated to meet a modest rangeof performance metrics (rutting, shifting, cracking) over a modest rangeof conditions (temperature, traffic load). Higher-performance ACC can beformulated with the addition of polymerizing agents; these give accessto much higher performance ranges and wider conditions, but they raisethe cost of the ACC significantly. More recently, aramid-fiberamendments and manufacturing methods (including specifically the ACEproduct from Surface Tech of California, which is described in U.S. Pat.Nos. 9,469,944 and 9,926,672) allow pavement engineers to accessintermediate performance and condition ranges, at costs that are alsomoderate. Embodiments of the present invention apply this new,intermediate-performance asphalt in a new role to improve theperformance (i.e., traffic and economic performance) and maintainabilityof an ACC paving system.

Embodiments of the invention improve the performance and longevity ofrepairs to damaged pavement (which may be Portland-cement concretepavement, asphalt-cement concrete pavement, or a prior asphaltresurfacing of PCC pavement). A damaged pavement surface is ground backfar enough to reach a portion that is still sound and/or useable as abase. Next, an aramid-fiber-reinforced interlayer of asphalt isdeposited. In a preferred embodiment, the aramid fiber is provided byintroducing a suitable dose of the ACE product from Surface Tech ofCalifornia into the interlayer asphalt mix; this mix is deposited at athickness of about 1″. Finally, an asphalt surface (travel) layer isdeposited over the interlayer. The thickness of the surface layerdepends on the traffic profile that the pavement is designed towithstand. Surface thickness may range from 1.5″ to 3.5″ or more.

Embodiments also recognize the impact of local material variations onthe performance and economic cost of a pavement repair system. Manycomponents of an asphalt cement concrete must be sourced locally to theinstallation—it is economically infeasible to ship sand, gravel andother bulk materials from a distant source to a plant where they will bemixed (or, equivalently, to ship the mixed asphalt from a distant plantto a road-repair location). However, the “gravel” or “sand” available inone location may have different properties from “gravel” or “sand” inanother location—for example, “No. 8 (2.36 mm)” gravel from one sourcemay differ in type, angularity, porosity and/or hardness from theidentically-named “No. 8” gravel from a different source. Similarly, thetypes of liquid binder available (and their costs) may vary from placeto place; and it may be important to be able to substitute a slightlydifferent (often cheaper) binder. In addition, each manufacturing plantperforms slightly differently, even with the same equipment andprocessing conditions. These differences may result in a recipe that wasproven in one location being nonconforming when manufactured elsewhere.Thus, local adaptations of a basic aramid-fiber-reinforced ACC recipemay be important to achieve optimal engineered performance at afavorable price.

FIG. 1 shows a side view or section of a portion of pavement constructedaccording to an embodiment of the invention. At the base (bottom), a bedcomprising a gravel or stone aggregate is shown (110). This is often apre-existing Portland cement concrete pavement, which may have cracks,damage, defects or other irregularities such as the expansion jointshown at 120. These irregularities allow different portions of the bedto move differently under load, as suggested by the misaligned,differently-sized arrows. Irregular motion may be characterized as“rocking slabs,” with poor load transfer from PCC slab to slab, which anengineer may seek to stabilize before repair, or may address with aninterlayer like the one described herein.

Over the bed 110, a reflective crack relief interlayer (“RCRI”) 130 isinstalled. The RCRI is a key portion of an embodiment of the invention,and will be described in greater detail below.

Over the RCRI, a hot-mix asphalt travel layer 140 is installed. The topsurface of the travel layer 150 is largely exposed and visible as thetravel surface (it is understood that the surface may be partly or fullycovered with paint, markers or other objects necessary for trafficcontrol or similar purposes). An embodiment may include a geotextilelayer, e.g. at 160 or 170, interposed between two adjacent layers, butthis is not an essential part of an embodiment. A pavement that omitssuch layers will still perform well, and will cost less to construct.

An embodiment helps correct a problem encountered when a hot-mix asphaltcement concrete travel layer is installed directly over a damaged orirregular bed, such as a legacy Portland-cement concrete roadway. FIG. 2shows a photo of a Portland cement concrete pavement, 210, and the samepavement after an asphalt cement concrete resurfacing layer (alone) hasbeen placed in service for six (6) months, 250. Note that a crack 260has appeared in the repaired surface; this crack corresponds to theexpansion joint 220 in the original Portland cement concrete bed. Thistype of damage is called “reflective cracking.” It results from stressesand strains on the repair surface caused by tension and horizontalmovement occurring between portions of the underlying bed.

An embodiment can improve the performance and longevity of areconditioned, restored or repaired pavement by proceeding in accordancewith the method outlined in FIG. 3. First, the existing pavement isprepared for further work (310). Preparations may include removing(e.g., by grinding) previous failed surfaces to reach a sound base, orfilling or patching holes in the existing surface. If the existingpavement is judged to be adequate as-is, preparation may be as simple assweeping or removing large debris.

In prior-art constructions, a geotextile sheet might be placed over theprepared pavement (320). But when an embodiment of the invention is inuse, the sheet is not necessary and can be omitted to save expense andconstruction time.

Now, a reflective crack relief interlayer (RCRI) is installed (330). TheRCRI is a modified asphalt-cement concrete material, comprising gravel,aggregate, sand, bitumen, optional modified polymer binders,and—significantly—aramid fibers to allow the RCRI to meet performancespecifications chosen to ensure proper performance and long life in viewof the loads that the finished restored pavement is designed to support.In a preferred embodiment, an aramid-fiber additive prepared asdescribed in U.S. Pat. No. 9,926,672 is used. In the preferredembodiment, the RCRI asphalt cement concrete is a plant-mixed materialwhere aramid fibers bound together by a meltable wax are introduced ator near the mix stage so that the fibers are distributed evenlythroughout the interlayer mix. In a preferred embodiment, the aramidfibers are at least 1½″ (38 mm) long. A mixture of fiber lengths may beused, but in this case also, a median fiber length of at least 1½″ ispreferred. Introducing the aramid amendment product into a ContinuousDrum Plant through the RAP collar, or into the weigh hopper of a BatchPlant, usually provides the best product consistency. Dosing all at oncewill not provide the best dispersion of aramid fiber throughout the RCRImaterial. A continuous, automatic metering system designed specificallyto measure and dispense asphalt amendments in the form and at theconcentrations appropriate for the ACE® product will improve confidencethat the bulk asphalt mix will match test samples whose propertiessatisfy engineering target ranges. The RCRI should be at least 1″ thick,but generally need not be much thicker than about 2″.

In an alternate prior-art construction, a geotextile sheet may be placedover the interlayer (340). But when an aramid-reinforced RCRI accordingto an embodiment is used, this sheet may also be omitted.

Finally, a conventional hot-mix asphalt travel layer is installed overthe RCRI (350). The thickness of the travel layer depends on the trafficload; for a project expected to serve up to 3,000,000 20-year EquivalentSingle Axial Loads (“ESALs”), the travel layer should be at least 1.5″thick. For 3M-10M ESALs, a 2.5″ travel layer is indicated. A 3.0″ travellayer can serve 10M to 30M ESALs, while a pavement serving even moretraffic should be at least 3.5″ thick.

When the travel layer is complete, it may be adorned with trafficmarkings, Botts' dots, rumble strips, or other surface features (360).

When the travel layer's service life has elapsed (typical design lifetargets are 5-10 years), the pavement may be restored by removing thetravel layer (i.e., back to the RCRI) (370), and repeating steps350-360. The RCRI may retain its structural and functional integrity fortwo or more restorative cycles before it, too, must be removed andreplaced.

Reflective Crack Relief Interlayer Compositions

The RCRI according to an embodiment is a modified plant-mix asphaltcement concrete material. Many of the ingredients are conventional, andconventional modifications to the recipe have the expected results. Inone embodiment, a basic mixture may include ⅜″ (4.75 mm) Nominal MaximumSize Aggregate (“NMAS”) with an asphalt content from about 7% to about10%, compacted to achieve an air-void percentage from about 0.5% toabout 2.5%. The mineral aggregate fraction (“VMA”) is preferably around16%, and the in-place density is about 96-98% of Gmm (maximum).

Conventional interlayer specifications for such a product would includehighly polymer modified liquid asphalt requirements as a criticalingredient, and a volumetric mix design. An ACC interlayer mixapplication is a razor-edge design that is focused on absorbing the highmovement of the PCC joint while being just stiff enough to not rut. Itis very difficult to achieve adequate performance at any point in aresurfacing project, and virtually impossible to achieve satisfactoryperformance over any significant distance or area.

An embodiment improves upon conventional ACC interlayer mixes by addingaramid fiber (e.g. via the ACE product from Surface Tech). This fiberreduces the need for expensive polymer-modified binders, and reducescracking seen in a flexural beam fatigue test when compared totraditional non-modified asphalt by over 90% at 600 microstrains and byover 200% at 900 microstrains. This is significant because mostbottom-up (reflective) cracking is in the 600+ microstrain range. Thearamid-modified ACC used in an embodiment is a new, plant-made andpaver-laid asphalt interlayer design that provides ten times the crackresistance of a conventional asphalt interlayer. Further, this ACC willmeet the expected performance criteria no matter where the product ismade, allowing the benefits of the present interlayer system to beenjoyed throughout the United States.

ACC interlayers according to an embodiment also allow for more accuratecontrol of pavement characteristics such as water permeability—thearamid fiber helps control water infiltration, so the finishedinterlayer may be impermeable or semi-impermeable. Controllingpermeability is important in high-rainfall areas, and in areas withexposure to freezing temperatures; keeping the water out of the base orfoundation of the pavement system will extend the life of the pavementsystem.

In a preferred embodiment, aramid fiber is introduced at a dosage fromabout 4 oz. per plant-ton of ACC mix, to about 10 oz. per plant-ton,with standard asphalt binder graded by the high and low temperatures ofthe project location. ACC used in an RCRI according to an embodimentshould fall into the following performance ranges:

Performance- Based Testing for Aramid Reinforced RCRI PurposePerformance Test Method Test Temp¹ Target Criteria Rutting Hamburg WheelTracker AASHTO T324 40, 45, or 50 C. 5,000-20,000 passes passes to ½″(12.5 mm) (optimal range rutting 7,000-15,000) Reflective IndirectTensile Cracking ASTM D8225 13, 20, or 25 C. 650-1,800 index CrackingTest (IDEAL-CT) or Repeated Flexural AASHTO T321 10, 15, or 20 C.20,000-40,000 cycles Beam Fatigue with polymer: 40,000-80,000 cycles¹Testing temperature is determined by climate. Rutting test temperatureis determined by PG 52, 58, and 64-xx 98% reliable climates,respectively. Cracking test temperature is determined by PG xx-34, 34,and 22 98% reliable climates, respectively.

In another aspect of the invention, embodiments can be tuned forparticular use cases by setting the aramid-fiber dose used in theinterlayer (RCRI) asphalt mix to target a range of beam-fatigue cycles.An asphalt that tests out to begin cracking between 20,000 and 40,000beam-fatigue test cycles is suitable for moderate to severe cracking ACCor jointed PCC applications with a service life of 10 to 15 years. Anasphalt that tests out to begin cracking between 40,000 cycles and80,000 cycles is suitable for moderate to severe cracking ACC or jointedPCC applications with a service life of 15 years or more.

Embodiments may also be characterized by their performance in HamburgWheel rutting tests, with a target range from 5,000 to 20,000 passes(ideally, targeting the optimal range of 7,000 to 15,000 passes). Whencharacterized using the Indirect Tensile Cracking Test (“IDEAL-CT”), anindex between about 650 and about 1,800 provides good results for anembodiment. Ultimately, the rutting test (Hamburg Wheel Rutting Test)and one cracking test (Flexural Beam Fatigue Test or IDEAL-CT Test)should be met in order to provide the desired performance for theembodiment.

Recipe Localization—Material Sourcing Considerations

As mentioned earlier, many of the ingredients of an Asphalt CementConcrete intended for use as a Reflective Crack Relief Interlayeraccording to an embodiment of the invention, must (for all practicalpurposes) be sourced locally to the pavement installation. And, becauseof differences between nominally-equivalent ingredients, it is oftenimportant to formulate and test ACC samples to ensure that thelocally-manufactured RCRI will perform in accord with engineeringprojections. A recipe that works in one location may performunacceptably in a different location having similar conditions, becausee.g. the gravel, sand, bitumen or other conventional ingredientavailable at the second location is different from the same-namedingredient that was used in the first location.

Thus, in one aspect, an embodiment of the invention comprises activitiesto develop a recipe to meet desired test-performance specifications,using a particular set of locally-available conventional ingredientsalong with the aramid-fiber reinforcements and optional polymer binders.FIG. 4 outlines this recipe-development process.

First, an engineer sets target performance specifications based onconditions and expected loads for a RCRI interlayer (410). An initialACC recipe is chosen (420), perhaps from a catalog ofpreviously-developed recipes, and perhaps as modified by the engineer'sjudgment. All ACC recipes of an embodiment comprise aramid reinforcingfibers, which are introduced into the ACC during mixing/manufacturing.

Now, a test batch of ACC is manufactured, using the ACC recipe andlocally-sourced ingredients (430). (The aramid reinforcing fibers arenot a heavy or bulky ingredient, nor are they used in large quantities,so they may be provided from any available source. Shipping and handlingof these fibers is not a significant cost driver.) The batch is testedas discussed above (440), e.g. by Hamburg Wheel rutting testing,IDEAL-CT crack testing, or flexural beam fatigue testing.

If the current ACC recipe does not meet the desired performancespecifications (450), then the engineer may reformulate the recipe touse different conventional ingredients, more or less aramid reinforcingfiber, or higher-performance polymer binders (460). Another test batchof the reformulated recipe is made (430) and tested (440).

Once an ACC recipe, made with locally-available ingredients, satisfiesthe project's performance specifications (470), the project can proceedwith the suitably-modified ACC RCRI recipe according to this embodiment.

The applications of the present invention have been described largely byreference to specific examples and in terms of particular structuralarrangements. However, those of skill in the art will recognize thatimproved pavement restoration can also be achieved by interlayer systemsthat include components different and/or beyond the aramid-modifiedreflective crack relief interlayer herein described. Such variations andimplementations are understood to be captured according to the followingclaims.

We claim:
 1. An asphalt cement concrete interlayer system forresurfacing a Portland cement concrete surface, comprising: a reflectivecrack relief interlayer (“RCRI”) comprising an aramid fiber-reinforcedasphalt cement concrete installed over a Portland cement concretesurface; and an asphalt cement concrete travel surface installed overthe RCRI.
 2. The asphalt cement concrete interlayer system of claim 1wherein the RCRI is installed directly upon the Portland cement concretesurface.
 3. The asphalt cement concrete interlayer system of claim 1wherein the asphalt cement concrete travel surface is installed directlyupon the RCRI.
 4. The asphalt cement concrete interlayer system of claim1 wherein the RCRI comprises plant-mix asphalt cement concrete dosedwith aramid fibers in a meltable wax binder during a mixing process. 5.The asphalt cement concrete interlayer system of claim 4 wherein theplant-mix asphalt cement concrete is dosed with aramid fibers at adosage of at least 4 oz. per plant-ton.
 6. The asphalt cement concreteinterlayer system of claim 4 wherein the plant-mix asphalt cementconcrete is dosed with aramid fibers at a dosage of no more than 10 oz.per plant-ton.
 7. The asphalt cement concrete interlayer system of claim1 wherein the RCRI is at least one inch (1″) thick.
 8. The asphaltcement concrete interlayer system of claim 1 wherein the RCRI is no morethan two inches (2″) thick.